The earliest implants for the treatment of OA of the knee consisted of fixed metallic hinges at one extreme, and some type of interposition at the other extreme. Interpositions included the use of fascia and other ‘soft’ biological materials, and also metallic tibial plateaus and metallic shells covering the distal femoral condyles. The ‘soft’ materials could fail due to inadequate strength and the lack of fixation to the bone. The metallic components fared better. Tibial plateaus, such as those in designs known as Macintosh and McKeever designs, served to space apart the bearing surfaces, thus potentially correcting the deformity, and provided a smooth bearing surface for the femoral condyles. Lack of fixation of the Macintosh implant to the tibia sometimes allowed movement or even dislocation, in the McKeever design, the use of keels prevented this problem. The femoral resurfacing devices faced the dual problem of matching the surface geometry of the original intact femur, and of shaping the distal femur to fit the implant. It is believed that obtaining a satisfactory range of motion, as well as stability, would be a problem in many cases due to the geometrical factors noted above. Another issue with such devices, which were not rigidly fixed to the bone, was that there would be ‘interface micromotion’ leading to resorption of the adjacent bone and replacement with fibrous tissue, leading to residual pain or aching.
This experience with interposition devices pointed to the benefits of rigid fixation of the device to the bone, and to geometrical compatibility within the joint. The question of whether pain resulted from the lack of fixation, or from the opposite side of the joint articulating with a rigid metal surface, was not clear. Clues to that question came from the hip, following the use of Austin-Moore implants for replacement of the femoral head. There was still some residual pain from uncemented femoral components, but far less when the components were cemented. There does not seem to have been a series of knees where a McKeever or a similar device has been fixed to the upper tibia using cement or other means, hence the source of the pain remains in some doubt in the knee. Another question with the use of an interposition device in the knee is the potential wearing away of the cartilage (or even bone) on the opposite side; because the rigidity of the metal caused the contact stresses to be elevated. In the case of a medial metallic tibial plateau, for a shallow bearing surface, the stresses would be significantly elevated, because in the intact knee, the meniscus would spread the load over a wide area. This fact suggests that the cartilage on the medial femoral condyle could wear out more quickly than in a normal healthy joint.
An implant design of interest was the Gunston, designed in the late 1960's by Frank Gunston from Winnipeg while working as a Fellow at John Charnley's Hip Center in Wrightington, England. A metal half-disc is embedded in the femoral condyle and just projects from it, and is articulated on a plastic runner set into the medial plateau. There was almost complete conformity in the frontal plane, and partial conformity in the sagittal plane.
This configuration had several benefits. The sagittal curve of the femoral condyle could be fairly closely reproduced given sufficient sizes, the slot in the femoral condyle gave a large surface area of strong cancellous bone for cemented fixation, and the tibial surface provided a combination of AP and rotational stability and laxity.
The negatives were that a single sagittal femoral radius could not reproduce the reduced radius in high flexion and the increased radius in extension at the distal end of the femur, cutting a slot in the femoral condyle sometimes endangered the strength of the bone on the outside, and the tibial plateau was of insufficient surface area such that sinkage and loosening occurred, and uncovered bone often impinged on bone on the opposite condyle or abraded against the plastic.
The polycentric knee, as it became known, was used in thousands of cases, especially at the Mayo Clinic, and provided good clinical results in a high percentage of cases.
In the early 1970's, Charnley produced an alternative implant, as shown, for example, in U.S. Pat. No. 3,953,899. Charnley used a thin flat metal plate with a single inner keel for fixation. This approach preserved of most of the strong cortical and sclerotic bone on the upper tibia to maximize the fixation, especially important for a component which did not necessarily cover the entire surface of the medial condyle. However it was difficult to position the flat plate so that it was in line with the force vector, which would result is some shear instability in some cases. Charnley also designed a plastic runner that was embedded into the distal femur. The name ‘Load-Angle Inlay’ (LAI) described this particular feature of Charnley's implant. The plastic runner was set so that it projected about 2 mm from the surrounding surface but was made to be flush at the anterior and posterior. This arrangement, where the plastic surface was convex and the metal surface was flat, was opposed to the convention of metal-plastic bearings, where the stationary and concave (or flat) component should be plastic and the moving surface metal. The rationale being that the stresses in the convex plastic would be higher potentially leading to delamination wear, and the plastic might wear unevenly which, in the extreme, might cause a discontinuity in the knee motion.
In practice, wear testing would be needed to determine whether the particular configuration used in the Charnley LAI would function well enough for its application, although there appears to be no public records in leading literature for such testing. Minns, Day, and Hardinge (1982) carried out a motion analysis of 29 patients, which indicated satisfactory function, with no mechanical problems being reported.
Another type of knee for medial OA was the unicompartmental or ‘uni’, introduced in the early 1970's. This design consisted of a metal femoral runner onlaid over the entire arc of the femoral condyle from extension to full flexion. The component design varied from having a curved undersurface to contact the femoral bone after removing any residual cartilage, to a facetted surface requiring flat cuts to be made with an osteotome or saw. The fixation was usually augmented with one or more posts, or blades, or a combination, using cement for immediate and long-term fixation. The tibial component consisted of a hemicircular disc of plastic, sometimes fitting inside a metallic baseplate. The baseplate helps prevent deformation of the plastic in the short and long-term, and the fixation to the bone was more durable. One disadvantage is that more tibial bone needs to be removed to account for the metal, paradoxically having an adverse effect on the fixation due to the fact that the strength of the cancellous bone in the proximal tibia diminishes with depth. As with the femoral component, fixation was by cement, and the undersurface had a combination of posts or blades. On all designs, the upper tibial surface has been close to flat, providing little AP stability, in contrast to the medial surface of the intact knee. This round on flat, or at best cylinder on flat, configuration produces high contact stresses. In long-term follow-ups, for net-shape molded polyethylene, there has typically been a trough formed due to wear and deformation, but no delamination.
An alternate design has been the meniscal bearing uni, where the femoral component had a spherical bearing surface, with the back surface being faceted. The tibial component consisted of a flat metallic plate with a polished upper surface. A plastic meniscus was interposed between the two components and conformed with each. This produced low contact stresses, which minimizes deformation and wear. There was no constraint to AP displacement, other than friction.
The following refers to a study carried out in one of the inventors' laboratory, on the nature of the osteoarthritic lesions at the time of total knee replacement surgery. One of the main purposes of the study was to determine if an early intervention procedure could have been carried out involving only replacement of the medial side of the joint, rather than a total knee. The study of 100 cases was reported to the Orthopaedic Research Society's Annual Meeting in 2007. The predominant lesion of the medial femoral condyle was distal, which is the region which undergoes weight-bearing in walking, by far the most frequent activity of everyday living. The posterior condyle on the other hand was frequently preserved, which makes sense because it is only weight-bearing in the less frequent high flexion activities such as rising from a chair and steep stair climbing. The lateral condyle was usually intact by visual appearance. A later study where the lateral histology was examined, showed that the cartilage structure was normal for that age group of individuals.
On the tibial side, the lesion on the medial plateau varied in location. On the other hand, the lateral side showed normal cartilage on that area covered by the meniscus, but cartilage with some softening and fibrillation on the area not covered by the meniscus. Hence the medial side showed degeneration where a repair was necessary, while the lateral side was frequently normal such that it could sustain normal weight-bearing without need of replacement.
Most of the lesions occurred within the anterior half, the central half, or extended more than one half. A lesser number involved the posterior. When all of the lesions for all 100 cases were superimposed, it was seen that all of the medial tibial plateau could be involved. This indicated that if a single style of tibial component was designed, it would need to cover substantially the whole of the tibial plateau.